The invention relates to an X-ray examination apparatus, comprising an X-ray source for emitting an X-ray beam, an X-ray detector which is arranged so as to face the X-ray source and which serves to form an X-ray image of an object to be arranged between the X-ray source and the X-ray detector, a power supply system which is connected to the X-ray source for the supply of current and voltage to the X-ray source, absorption means arranged between the X-ray source and the X-ray detector to attenuate the X-ray beam, and an image processing unit which is connected to the X-ray detector in order to store the X-ray image as absorption values arranged in a matrix.
An X-ray examination apparatus of this kind is known from European Patent Specification EP-B1-157 688 which corresponds to U.S. Pat. No. 4,670,896.
The cited Patent Specification discloses that a substantial difference in contrast can occur in an X-ray image formed by exposure of an object to X-rays. These brightness differences in an X-ray image can arise because a part of the X-rays does not penetrate the object to be examined and is incident directly on the X-ray detector, or because the object to be irradiated exhibits substantial differences in absorption. For example, when in medical diagnostics an organ exhibiting a high absorption, for example a heart, is surrounded by organs which are compartively transparent to X-rays, for example lungs, an X-ray image is obtained in which the contrast within the organ of interest is low in comparison with the contrast between the brightest and the darkest areas in the overall X-ray image. In order to make the dynamic range of the X-ray image coincide as much as possible with the contrast between the brightest and the darkest areas in the organ of interest, absorption means are arranged in known manner between the X-ray source and the object to be irradiated. To this end, there is made a first X-ray exposure which is projected as an optical image onto the object, via an X-ray image intensifier, a television camera device which cooperates with an exit window of the X-ray image intensifier, and a projection device. Subsequently, the absorption means, having an optical absorption proportional to their absorptive power for X-rays, are manually introduced into the light beam of the projected X-ray image, thus achieving a desired reduction of the dymamic range of the X-ray image. Such a method for positioning the absorption means necessitates the use of an additional projection device, increasing the complexity of the X-ray examination apparatus, and is comparatively cumbersome because of the manual operations involved. Moreover, because of the difference in interaction of X-rays and light with matter, a desired X-ray attenuation is merely approximated when the absorption means occupy a position producing a desired optical attenuation.